PURPOSE: To prevent easily a steel sheet from discoloring by spraying a chelating agent on the steel sheet drawn out from a pickling vessel and washing it with water in such case that a pickling line is stopped or slowly moved.

CONSTITUTION: Hydrochloric acid content is wrung from a steel sheet P which is drawn out from a pickling vessel 1 with the wringer rolls 2 and the hydrochloric acid content is removed by passing successively it through a spray vessel 5 provided with the water spray nozzles 3 and a deflector roll 4 and a hot water vessel 8 provided with the deflector rolls 6 and the wringer rolls 7. In this pickling line, an aq. chelating agent soln. is sprayed by making the spray group of the front stage of the spray vessel 5 the spray nozzles 9 for a chelating agent in a period of slow movement and a period of stop of the line. Thereby Fe ion of an ionic state is changed into a water-soluble chelate compd. and the steel plate can be easily washed with the water spray nozzles 3 and the discoloration is not caused in stopping for several minutes.